Lab talk

Researchers based in the US and Korea have characterized the carrier mobility of graphene produced via the thermal decomposition of SiC – a method that allows large areas of graphene to be grown. The scientists examined graphene grown on Si- and C-faces of the material in a study that involved the construction of test transistor structures. Samples were either capped directly by an HfO2 dielectric layer, or coated with PVA prior to HfO2 deposition to reduce the interface effects between the graphene layer and the dielectric film.

PVA was chosen as a buffer layer because it contains both the polar moieties (C–O bonds) capable of bonding to the polar HfO2 and the nonpolar moieties (C–H bonds) to interface with non-polar graphene. Also, a linear PVA chain makes it possible for its C–O bonds to orient towards the HfO2 en masse while its C–H bonds orient towards the graphene. Finally, the oxygen atoms in PVA are able to bond to the Hf atoms at the interface, which, to first order, closely mimics the bonding structure of Hf atoms in the HfO2 bulk.

In each case the mobility of Si-face and C-face graphene increased after incorporating PVA/HfO2 dielectrics. The mobility of Si-face graphene (~90%) increases much more than that of C-face graphene (~20%). The researchers speculate that the mobility improvement arises from the suppression of random potential fluctuations at the HfO2–PVA interface by chemically passivating the Hf centers.

The groups based at Cornell University, The University of Wisconsin-Stevens Point and Hanyang University are working towards implementing high-speed graphene devices on large-scale substrates.

About the author

This research was conducted by teams from the school of electrical and computer engineering at Cornell University in Ithaca, NY, US, and Hanyang University in Seoul, Korea (Prof. Jo-Won Lee). Graphene teams at Cornell have been supported by the Air force MURI program and the NSF (Cornell CNS). Dr Moonkyung Kim and his group, the head of which is Prof. Sandip Tiwari, are conducting research on nanoscale electronic devices such as nonvolatile memories, mechanical switching devices, flexible electronics and carbon-based devices. Dr Kim fabricated the graphene transistors and analysed the electrical properties in this study. Dr Jeonghyun Hwang is in the research group of Prof. Michael Spencer and they are actively working on graphene and other two-dimensional material systems, such as BN and MoS2. They performed the growth and characterization of epitaxial graphene on SiC in this work. Dr Lori Lepak was a member of Prof. Spencer’s lab at Cornell and is now doing her postdoctoral research at the University of Wisconsin at Stevens Point.